Wafer-to-wafer bonding is an increasingly important packaging technique. Packaging typically is a major cost component of modem microelectronic devices. Wafer-level packaging provides an opportunity to achieve significant cost reductions by packaging various sensors, actuators, and/or circuits together in wafer form, thereby eliminating the cost of packaging the various chips separately. In addition, the bonding of one wafer to another can be used for various purposes other than just cost reduction. Applications of this technology include use with delicate circuits or micro-electromechanical system (MEMS) that need to be protected from particles or from mechanical damage.
Various techniques have been used to effect the bonding of one wafer to another. Generally these techniques can be classified as either (1) a direct bond, (2) an anodic bond, or (3) an intermediate layer bond. In a direct bond, wafers are placed in direct contact with each other. Such bonding techniques rely on the tendency for one smooth surface to adhere to another. The bonding occurs without significant pressure, electric fields, or intermediate layers of other materials. However, thermal annealing is used after the wafers are placed in contact with each other. For anodic bonds, the wafers to be bonded are placed in contact and then an electric field is applied between the two wafers coupled with an elevation in temperature generally to the range of 300-450 C. This type of bond is typically formed between a sodium-baring glass wafer and a silicon wafer. For intermediate layer bonds, an intermediate layer is required to form the wafer-to-wafer bond. Materials used in such bonds include eutectics, polymers, and solders. For polymers the current practice is either to place some material or adhesion promoter onto the surface which improves the bonding mechanism between the polymer and the surface the polymer is being placed. Often the surface is roughened up to increase the surface area. Generally, the adhesion of the polymer is only marginally improved.
In other applications, the bonding of multiple liquid polymer layers together requires special curing processes, adhesion promoters, or geometry configurations to achieve acceptable adhesion. Typically, the adhesion of one polymer to another has been found to be either marginal or unacceptable.